Technically Speaking

By Bill Nadeau

Gas management rules have got to be one of the top debated issues within the technical community. In actual fact determining how much gas to take with you is quite simple if you employ a responsible management attitude. First you must begin by determining your gas consumption rate. This needs to be done in an environment that is as close to the conditions that you will be diving in during actual technical dives. Usually I select three ATA (66 FSW) and swim at a normal pace for a suitable period of time (around 20 minutes). I record my starting and ending pressures and complete my calculations on the surface (see example in figure 1). Ultimately I will want to calculate how many PSIG I consume per minute on the surfacethis is what is referred to as a SAC Rate or a Surface Air Consumption Rate.

Once calculated the SAC Rate can be applied to any depth during any dive as long as you maintain the same breathing rate and cylinder size. Technical divers will convert this value to an RMV or Respiratory Minute Volume which allows them to calculate their consumption rate on any size cylinder(s) because the units are based on volume rather than pressure which is subject to the cylinder configuration (see figure 2).

Once your RMV is calculated you can adjust your dive plan based on the amount of gas you can expect to use and then plan for the total amount of gas you need. The caving community and efforts of great pioneers like Sheck Exley devised a standard that has now become the basis for all gas management rulesit is called the rule of thirds. Its premise is that a dive is turned after 1/3 of the diver's gas is consumed. It allows a cave diver to use a third of his gas to swim into the cave, a third to swim out of the cave and a third for any problems that might occur during the dive. The rule is based on exiting the dive with at least one third of your gas left. To many it may appear conservative but when things go wrong, one third of your gas is not a lot to get you out of trouble. There have been some arguments that propose the rule should be modified accordingly for open water dives or dives that do not require a diver to return to the same point of entry. These issues are discussed in all technical training courses and illustrate how complex managing a dive plan can get.

The management role does not stop there. Next the diver needs to determine the size of tanks needed to hold enough gas required to complete the dive and decompression safely. What are the tank's buoyancy characteristics when empty? Have they been cleaned and properly labelled for the gas they will be filled with? And then the diver needs to start reviewing each piece of gear necessary to support the dive while not task loading himself (refer to Gadget Pandemonium: The Redundancy Dilemma, July/August 1997 issue of DIVER Magazine). This entire subject represents a huge component of technical diver training and can be referred to as Managing your Gas and Managing your Equipment.

Our management skills will next be put to work taking care of all of the contingency plans that need to be sorted out. As the complexity of the dive increases so does the riskwe have already talked about this. Managing the Risk (and reducing it) involves Managing the Contingencies that will cover our behinds when things go wrongor better yet preventing things from going wrong in the first place. With each extra piece of gear, each foot deeper we go, each foot farther into a cave or a wreck and with each extra minute of deco we accumulate, we have to explore and consider all of the things that can go wrong and what we can do to prevent or resolve them.

How about Mother Nature herself? She is the one component of a dive we have the least control over and she deserves the greatest amount of respect. Managing the Environment includes continuous updates and evaluations and asking ourselves whether any new developments have put the dive outside of our limitations. This requires a lot of research and experience.

Planning a dive means managing a number of factors. At the technical level, divers will adjust their plan many times, balanc ing one limit against another before deciding on a final strategy. Experience, training and good judgement allows them to do this. A good technical diver will not compromise safety or increase the level of risk to make a dive possible, they will compensate and prepare for itthat's what managing is all about.